The present invention relates to a process for producing gallium-containing solution having a high concentration of a gallium component from aluminum smelting dust.
Gallium is used as GaAs for high-speed semiconductors for satellite correspondence, IC for high-speed computers, GaAs or GaP for semiconductor lasers for optical communication and mass magnetic bubble domain memories using gallium gadolinium garnets.
Gallium is widely distributed all over the earth, but does not exist as highly concentrated ore. Therefore, gallium is presently obtained by the following processes
(i) Gallium is recovered from Bayer liquor which is a highly concentrated alkaline solution dissolving Al(OH).sub.3. An electrochemical reduction process with a mercury cathode or a solvent extraction process is used for recovering gallium from the Bayer liquor. The solvent extraction process developed by Rhone-Poulenc Co. using KELEX 100 (trademark of alkylated-8-hydroxy-quinoline) is well-known (See pages 439-442, Vol. 2, 1979, Proceeding of the International Solvent Extraction Conference and French Patent No. 7629009). PA0 (ii) Gallium is recovered from aluminum smelting dust by flotation (Japanese Patent Publication No. SHO59-111919, June 28, 1984) or alkaline roasting (See British Pat. No. 1527981). PA0 (iii) Gallium is recovered from the residue of a zinc leach solution in a zinc smelting process, in which the residue is leached with sulfuric acid. And gallium is recovered by an extracting agent such as versatic acid or isopropyl ether (See pages 65-76, Proceeding of Fourth Joint Meeting MMIJ-AIME 1980. TOKYO).
Though gallium is recovered from residue of a zinc leach solution (iii) in Japan, the major source of gallium is bauxite which is the raw material for making aluminum. However, the efficiency for recovering gallium directly from bauxite is low, because of the low gallium content in bauxite, e.g., 50-100 p.p.m.
The process of recovering gallium from Bayer liquor is mentioned in process (i) above. Electrochemical reduction with a mercury cathode create an environmental pollution problem because of the mercury. The solvent extraction process also has problems such as deterioration of the expensive extracting agent, loss by decomposition and the suspension and contamination of the Bayer liquor with organics, because the Bayer liquor is highly caustic.
The complicated processes and expensive treating costs were required in process (i) because of the very low gallium content. Aluminum smelting dust having comparatively high gallium content is preferable as a raw material. Gallium contained in Bayer liquor exhibits the same behavior as aluminum, and almost all of the gallium goes into alumina. The gallium concentration in alumina depends on the quality of bauxite, and 20 p.p.m.-100 p.p.m. is usual. In aluminum smelting dust, gallium is concentrated as highly as 800 p.p.m.-3000 p.p.m. The general composition of dust evolved from an aluminum electrolysis process is 0.08-0.30 wt.% of gallium, 0.5-2.0 wt.% of iron, 10-20 wt.% of aluminum, 10-20 wt.% of sodium, 10-30 wt.% of flourine, 10-30 wt.% of carbon and less than 1 wt.% of silicon, nickel, titanium calcium, copper, magnesium, cobalt and vanadium. For recovering gallium from the dust, first of all, gallium is extracted from the dust into an aqueous solution, and secondly the solution is treated so as to separate gallium from the other metal impurities.
The above-mentioned British Pat. No. 1527981 relates to a process for producing gallium from aluminum smelting dust.
The process consists of roasting the dust at a high temperature, after adding alkaline flux, followed by leaching the roasted mixture of dust and flux with water, and then precipitating the dissolved gallium by adding base metal powder (aluminum or magnesium) into the leach liquor. However, the above-mentioned process has a practical limitation, for the process requires not only special facilities for roasting, but expensive alkaline flux as well as fuel and power for combustion.
The direct leaching of the unroasted dust with either mineral acid such as sulfuric acid, hydrochloric acid and nitric acid or alkali such as sodium hydroxide and potassium hydroxide is also unpractical, as gallium is hardly leached out.